1. Field of the Invention
The present invention relates to a catalytic process for converting synthesis gas to hydrocarbons.
2. Description of the Prior Art
The reaction to convert carbon monoxide and hydrogen mixtures (so called syngas) to higher hydrocarbons over metallic catalysts has been known since the turn of the century. This reaction is commonly referred to as the Fischer-Tropsch or F-T synthesis. The F-T synthesis was exploited commercially during WWII in Germany. By 1944 a total of nine F-T plants were operating in Germany, primarily using a catalyst composed of cobalt, magnesium oxide, thorium oxide, and kieselguhr, in the relative composition 100:5:8:200. Later, most of the thoria was replaced by magnesia, primarily for economic reasons. Currently, commercial Fischer-Tropsch plants, which use a precipitated iron-based catalyst which contains various promoters to improve the stability and product distribution, are operating in South Africa.
The common F-T catalysts are nickel, cobalt, and iron. Nickel was probably the first substance to be recognized as capable of catalyzing the reaction of syngas to hydrocarbons, producing mainly methane (see, for example, R. B. Anderson, The Fischer-Tropsch Synthesis, Academic Press (1984), p. 2). Iron and cobalt are able to produce higher chain hydrocarbons and are, thus, preferred as catalysts for the production of liquid hydrocarbons. However, other metals are also capable of catalyzing the conversion of synthesis gas. Among the Group VIII metals, ruthenium is a very active catalyst for the formation of hydrocarbons from syngas. Its activity at low temperatures is higher than that of Fe, Co, or Ni, and it produces a large amount of heavy hydrocarbons. At high pressures, it produces a large amount of high molecular weight waxes. Other metals which are highly active, such as rhodium, yield high amounts of oxygenated materials (see Ichikawa, Chemtech, 6, 74 (1982)). Osmium has been found to be moderately active, while Pt, Pd, and Ir exhibit low activity (see Pichler, Advances in Catalysis, vol. IV, Academic Press, N.Y. (1952), R. B. Anderson, The Fischer-Tropsch Synthesis, supra and Vannice, Journal of Catalysis, 50, 228-236).
Other metals that have been investigated include rhenium, molybdenum, and chromium, but these exhibit very low activity with most of the product being methane (see R. B. Anderson, The Fischer-Tropsch Synthesis, supra).
Various combinations of metals can also be used in the F-T process. Doping cobalt catalysts with nickel causes an increase in methane production during F-T synthesis (see Catalysis, vol. IV, Reinhold Publishing Co., (1956), p. 29). In U.S. Pat. No. 4,088,671 to T. P. Kobylinski, entitled "Conversion of Synthesis Gas Using a Cobalt-Ruthenium Catalyst", the addition of small amounts of ruthenium to cobalt is shown to result in a higher overall activity and lower methane production during F-T synthesis than using cobalt alone. Thus, these references teach that combinations of two or more F-T active metals can result in an active F-T catalyst with characteristics which are similar to the combined characteristics of each of the individual components.
Combinations of cobalt with non-F-T active metals have also been reported for the conversion of synthesis gas to specific products and, in some cases, at specific conditions. In Nakaoji, U.S. Pat. No. 3,988,344, the combination of cobalt with a second Group VIII metal and tungsten is claimed for the enhanced production of methane from synthesis gas. Knifton, U.S. Pat. No. 4,390,734 and Japanese Kokai 57/130932 describe the combination of Co and Rh for the production of oxygenated products, such as glycols or aldehydes. Fischer-Tropsch catalysts consisting of combinations of cobalt with either platinum or palladium, supported on a variety of solids, including alumina, have been reported by Sapienza et al., U.S. Pat. No. 4,396,539. These catalysts, however, relied on preparation from the metal carbonyls in order to form solid solutions on the surface of the solid support and were distinguished by an x-ray impermeable layer covering the support, thereby resulting in a catalyst exhibiting a unique x-ray diffraction pattern in which the structure of the solid support was completely masked by the metallic components. In particular, when the catalysts of the Sapienza et al. patent are supported on alumina, they are distinguished by the complete absence of any x-ray diffraction peaks in the 2 .theta. range of 65 to 70 degrees. In x-ray diffraction .theta. equals the angle of refraction.
Combinations of metals with certain oxide supports have also been reported to result in improved hydrocarbon yields during F-T synthesis. The use of titania to support cobalt or cobalt-thoria is taught in Payne et al., U.S. Pat. No. 4,595,703, entitles "Hydrocarbons from Synthesis Gas". In this case the support served to increase the activity of the metal(s) toward hydrocarbon formation. In fact, titania belongs to a class of metal oxides known to exhibit strong metal-support interactions and, as such, has been reported to give improved activity for a number of metals during F-T synthesis (see for example, M. A. Vannice, Journal of Catalysis, 74, 199 (1982)). Combinations of titania and two or more metals have also been shown to yield improved F-T activity. In Mauldin, U.S. Pat. No. 4,568,663, the use of combinations of cobalt and rhenium, or cobalt, rhenium, and thoria, supported on titania is claimed as useful for the production of hydrocarbons from methanol or synthesis gas.
In a series of European patent applications (EP 110449, EP 142888, EP 167215, and EP 188304), Shell International described an improved F-T catalyst comprised of cobalt promoted by at least one of the metals in the group consisting of zirconium, titanium, and chromium, preferably supported on silica, alumina, or silica-alumina. The addition of Group VIII noble metals to zirconia-promoted cobalt catalysts was claimed in European patent application EP 221598, which teaches improved activity upon addition of platinum to a cobalt catalyst already promoted with zirconia. Thus, the Shell work shows that the addition of a Group VIII metal to a Fischer-Tropsch catalyst is only useful when the catalyst already incorporates a well known promoter, such as zirconia, as a main component of the catalyst.